Sen. Jon Kyl (R-Ariz.) will resign at the end of this year, forcing Republican Gov. Doug Ducey to appoint a new replacement to the late Sen. John McCain's seat ahead of a 2020 special election to fill the last two years of McCain's term.

Kyl sent Ducey a letter Wednesday announcing his resignation effective at the end of this month.

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"When I accepted your appointment, I agreed to complete the work of the 115th Congress and then reevaluate continuing to serve," Kyl wrote. "I have concluded that it would be best if I resign so that your new appointee can begin the new term with all other Senators in January 2018 and can serve a full two (potentially four) years."

Ducey's office said in a statement that the governor will appoint another replacement in the near future. Prospects include GOP Rep. Martha McSally, who lost a campaign for Arizona's other Senate seat earlier this year; Kirk Adams, Ducey's former chief of staff and a former speaker of the Arizona House; and Maricopa County Attorney Bill Montgomery.

"Senator Kyl didn't need to return to the Senate. His legacy as one of Arizona's most influential and important political figures was already without question," Ducey said. "But he did return and I remain deeply grateful for his willingness to step up and serve again when Arizona needed him."

The new appointed senator must be a Republican, according to state law, because McCain won the term as a Republican in 2016. But beyond that, the Arizona GOP is divided over the best choice for the appointment.

While McSally brings experience, connections and name identification from her recent statewide campaign, she also became the first Republican to lose a Senate race in the state in three decades when Democratic Rep. Kyrsten Sinema defeated her by 2.4 percentage points. Some Arizona Republicans have questioned the McSally campaign's decision-making and how far behind Ducey she ran as the governor coasted to reelection, casting doubts on whether she would be the best choice for an appointment.

Adams, who led the state House and narrowly lost a Republican primary for Congress before joining Ducey's office, has strong connections with the donor community in Arizona. But he has never run a statewide campaign. And Montgomery, who has been Maricopa County attorney since 2010, has no federal or statewide campaign experience.

But Montgomery has been elected multiple times in Maricopa County, where he turned aside a well-funded challenger in 2016 even as Republican Maricopa County Sheriff Joe Arpaio lost his reelection race. Though Maricopa — Arizona's largest county and home to Phoenix — is a traditional Republican stronghold, Sinema defeated McSally by more than 4 percentage points there, a significant factor in her victory.

Dan Eberhart, a Phoenix-based Republican donor, said there is "momentum" in the donor community for an "anybody but McSally" appointment to the seat.

"Donors are quietly whispering in Gov. Ducey's ear that Martha McSally had her chance and came up a little short," Eberhart said.

Barrett Marson, a veteran Republican strategist who ran a pro-McSally super PAC during the Senate race, said he expected the choice would come down to Adams or McSally.

"Each has incredible upsides and each frankly have some downsides," Marson said, pointing to McSally's recent loss and Adams' lack of statewide experience. He said the appointment would likely be decided based on who would have the best chance to hold the seat in the 2020 special election, when Democrats are expected to make a strong challenge, and again in 2022 when the seat is up for a full term.